Chair back



FIG. 1 is a front elevational view of a chair back in accordance with our design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a rear elevational view thereof, with the right-side elevational view being a mirror image;

FIG. 4 is a top plan view thereof;

FIG. 5 is a bottom view thereof; and,

FIG. 6 is a perspective view thereof with a chair, which forms the environment of the chair back, being illustrated in broken lines, which broken lines are for illustrative purposes only and form no part of the claimed design. 

The ornamental design for a chair back, as shown and described. 